This invention relates to a method and means for measuring certain parameters of the statistical distribution of a collection of particles. More particularly the invention relates to the measurement of the mean of the area distribution, the mean of the volume distribution, and the standard deviation of the area distribution of the particles.
This invention has usefulness in the measurement of the three above mentioned statistical parameters in a number of applications. For example, it would be useful in measuring the progress of the finish grinding of Taconite ore or cement, also in explosion hazard monitoring and dust respiration measurements as well as in measurements of bacteriologicals.
There has previously been developed a method and means for measuring the total particle volume or mass by the use of a monochromatic laser light beam directed through a collection of particles and then through a spatial filter which serves to establish a third power relationship between the diffracted light flux from each particle transmitted by the spatial filter and the radius of the particle. In this connection reference should be made to U.S. patent application Ser. No. 403,288 of William Leslie Wilcock, filed Oct. 3, 1973.
Other prior art devices have utilized computation of distribution data by the use of digital computers in systems which laboriously examine individual particles from the collection to be analyzed and use counting procedures to obtain statistics on the aggregate collection.
The method and means of this invention, like that of Wilcock relies on the fact that the total flux diffracted by a particle (the forward scattered light) is proportional to the square of the particle diameter while the angular scale of the Fraunhofer interference pattern in the Fourier or Fraunhofer plane of the lens collecting the diffracted light is proportional to the reciprocal of the particle diameter in such a way that the proportionality of the total diffracted flux to particle diameter varies from a fourth power relationship near the center of the pattern to a second power relationship for the whole pattern. Thus it is possible, as has been demonstrated by Wilcock, to use a spatial filter in the Fraunhofer plane such that the transmissivity of the filter changes as a function of radius or distance from the optical axis so that the total flux transmitted by the filter is proportional to any power of the particle radius from the second power to and including the fourth power.
It is an object of this invention to provide a method and means for measuring the mean of the particle distribution by area, the mean of the particle distribution by volume and the standard deviation of the area distribution by the use of spatial filters of the type taught by Wilcock.